Permeation of one component of a multi-component fluid mixture through a permeable membrane is related to the solubility coefficient, which describes the ability of the membrane to extract that component from the feed mixture, and the diffusion coefficient, which describes how fast that component will transport across the thickness of the membrane. These coefficients are constant for a given compound/membrane system for a given solvent and at constant temperatures.
Permeable membranes are used in a variety of contexts to separate a component of interest from a multi-component feed mixture. For example, in applications where a multi-component feed mixture has a single membrane-permeable component, that component is separated from the feed mixture by exposing the feed mixture to a permeable membrane. Since only the one component is membrane-permeable, that component is separated from the feed mixture and is collected from the other side of the membrane. In a separation process of this type, recovery of the permeating component or she "permeate" (i.e., the fluid which permeates the membrane) in any given time period is maximized by operating the membrane separation process at steady state permeation. Steady state permeation means that the rate of permeation of the permeating component essentially does not change with time. In addition, during steady state permeation the total throughput of the permeating component is maximized.
When a multi-component feed mixture having more than one membrane-permeable component is introduced to a permeable membrane, the resulting permeate will consist of all the membrane-permeable components from the multi-component feed mixture. Under steady state conditions, maximum throughput of the membrane permeable components will be realized and the proportion of each of those components in the permeate will be directly proportional to the rate of steady state permeation of each component. That is, when a multi-component feed which has two membrane-permeable components is introduced to a permeable membrane, two membrane-permeable components are separated from the other components of the feed mixture. The relative proportions of each of those two components in the permeate when the system is operated at steady-state is dependent upon the rate of steady-state permeation of each component.
The known membrane separation processes do not provide relative separation of individual membrane-permeable components vis-a-vis one another apart from the separation which occurs due to the differences in the rates of steady state permeation of the components.
Summary of the Invention
It is oftentimes desirable to provide a greater degree of separation of two or more membrane-permeable components relative to one another than is currently possible using known steady state membrane permeation. This need arises in both chemical analysis and other contexts. For example, in mass spectrometry, it is desirable to minimize the signal of the interferent relative to the analyte. This is possible by maximizing the proportion of the analyte relative to the interferent in the permeate from a permeable membrane. Additionally, in purification processes, it is often the case that two or more components in a multi-component fluid mixture are membrane-permeable and need to be separated from one another. The known membrane separation processes described above do not solve the problem of maximizing the separation of one membrane-permeable component relative another membrane-permeable component in a multi-component fluid mixture.
The present invention is directed to a membrane separation process, and the associated apparatus, which provides separation of one membrane-permeable component from another where both are contained in a multiple-component fluid mixture. The process and apparatus of this invention are equally well suited to multi-component fluid mixtures that are liquid or gaseous.
According to the invention, one embodiment of a membrane separation process is operated under non-steady state or dynamic conditions to provide increased selectivity in separating membrane-permeable components relative one another. The process, by operating under non-steady state conditions, takes advantage of the differential in the rates of attainment of steady-state permeation and the rates of fall-off from steady-state permeation of membrane-permeable compounds. The differentials in the rates of attaining steady-state permeation and fall-off from steady-state permeation by permeating compounds is based in large part upon the differences in the diffusivities of the permeating compounds.
When a feed stream containing two membrane-permeable compounds is exposed to a membrane, the permeation of each compound increases from an initial base line value to its steady-state value. This initial region of a permeation response curve is termed "sorption". "Desorption," which is the sorption process reversed, is where the introduction of the feed to the membrane is discontinued and the permeation response curve falls off from its steady-state value to a base line value. During the sorption process, compounds which have a faster rate of attainment of steady-state permeation, i.e., faster diffusing compounds, are enriched in the permeate over slower diffusing compounds, relative to the enrichment that is possible under steady-state conditions. Likewise, compounds which have a slower rate of attainment of steady-state permeation and therefore a slower rate of fall-off from steady-state permeation, i.e., slower diffusing compounds, are enriched in the permeate over faster diffusing compounds during the desorption process.
Sorption and desorption, by their nature, represent dynamic, non-steady state conditions. For this reason, neither sorption nor desorption can be maintained indefinitely. Thus, for the enhanced separation of membrane-permeable components from a feed stream relative one another, a continuous process requires a discontinuous feed stream.
In a process exploiting sorption, the feed stream is preferably interrupted after sufficient time has elapsed for one or more faster diffusing membrane-permeable components to approach or reach steady state permeation of the membrane, but prior to a slower diffusing compound or compounds reaching steady state permeation of the membrane. The permeate is collected during the period of sorption, and, in this manner, the differential in the rates of attainment of steady state permeation of the membrane-permeable components is exploited to produce a permeate enriched in a faster diffusing compound.
In a process exploiting desorption, the permeate is collected beginning at a time when the feed stream, which is at or near steady-state permeation or at some non-baseline permeation, has been interrupted to thereby exploit the differential in the fall-off rates from steady-state permeation to a base line value of the membrane-permeable compounds. The permeate collected is enriched in the slower diffusing compounds.
In one preferred form of the process of the present invention, a multi-component fluid mixture feed having at least two membrane-permeable components, wherein a first component approaches steady-state permeation of a membrane at a faster rate than a second component, is introduced to a first side of a permeable membrane for a predetermined period of time to selectively separate the first component from the second component. The predetermined time period is preferably less than that required for the second component to reach steady-state permeation of the membrane since that results in the greatest degree of enrichment of the first component. After the predetermined time period has elapsed, the introduction of feed to the first side of the membrane is discontinued or interrupted. The permeate enriched in the faster diffusing component is collected from the second side of the membrane while the feed is in contact with the membrane. It will be appreciated that the introduction and interruption of the feed, as described, can be continuously cycled to produce any desired amount of permeate. Alternatively, the process can be run as a batch process, i.e., only one feed introduction/interruption cycle.
In a continuous process, the multi-component feed is introduced to the first side of a permeable membrane and intermittently interrupted at selected time intervals such that the entire feed mixture never reaches steady state permeation of the membrane. An enriched permeate stream is collected from the second side of the membrane during the time intervals when the feed is in contact with the membrane. Thus, virtually any desired amount of permeate can be collected. Where a batch process is employed, the introduction of the multi-component feed is discontinued after the predetermined time period has elapsed and an enriched permeate stream is collected. This type of process is suitable in certain chemical analysis contexts where a relatively small permeate sample is all that is required.
Both the continuous and batch embodiments of the process of the present invention are suited to exploit the sorption and/or desorption portions of the permeation response profile for the feed mixture. That is, the permeate can be collected during sorption to obtain a mixture enriched in the faster diffusing compound or compounds or the permeate can be collected during desorption to obtain a mixture enriched in the slower diffusing compound or compounds.
Intermittent interruption of the feed stream is accomplished in one of several preferred manners. In one embodiment, an inert gas is introduced into the feed stream at selected time intervals to interrupt and "segment" the feed into discrete amounts so as to control and maintain the residence time of each "segment" of feed in contact with the membrane to the preselected time, which time is less than that required for the second membrane-permeable component to reach steady-state permeation. In another preferred embodiment, the feed stream is interrupted intermittently and "segmented" by the introduction of an inert liquid at selected time intervals so as to control and maintain the residence time of each "segment" or feed in contact with the membrane to the preselected time, as stated above. In an additional preferred embodiment, the feed is intermittently interrupted and "segmented" physically by a solid barrier or surface, such as a piston, which alternately covers and uncovers the first surface of the membrane, to control and maintain the residence time of each "segment" of feed with respect to the membrane. In another preferred embodiment, the feed stream flow to the membrane is discontinued and the membrane is evacuated with a vacuum for a preselected time, and thereafter the feed is reintroduced to the membrane and the process is repeated. Thus, by employing any one of the above-described means for periodically interrupting and segmenting the feed mixture and choosing the appropriate time interval for the feed to be in contact with the membrane, the permeation process continuously cycles through the sorption-desorption scheme and a permeate enriched in a faster diffusing compound is collected.
One preferred embodiment of the apparatus for carrying out the above-described process comprises a membrane that is permeable to at least two components of a multi-component fluid system in which the two components have differing rates of attainment of steady state permeation through the membrane. The membrane has first and second sides. The apparatus further comprises means for introducing a feed stream of the multi-component fluid system to the first side of the membrane; means for intermittently interrupting the introduction of the feed at preselected time intervals such that one component approaches steady state permeation of the membrane more rapidly than other components prior to each interruption of the feed; and means for collecting permeate enriched in one membrane-permeable component from the second side of the membrane. The feed interrupting means may comprise a valving mechanism which is operated to intermittently interrupt the feed, or a cylinder and piston combination in which the piston is repositionable to alternatingly cover and uncover the first side of the membrane, and thereby "segment" the feed.